1. Field of the Invention
The present invention relates to a projection optical system and an exposure apparatus comprising the projection optical system. For example, the present invention relates to a reflective projection optical system which is suitable for an X-ray projection exposure apparatus which transfers a circuit pattern on a mask onto a photosensitive substrate by a mirror projection scheme using X-rays.
2. Description of the Related Art
Exposure apparatus used for manufacturing semiconductor devices and the like have conventionally projected and transferred circuit patterns formed on a mask (reticle) onto a photosensitive substrate such as wafer through a projection optical system. The photosensitive substrate senses light upon the exposure and projection through the projection optical system, thereby yielding a resist pattern corresponding to a mask pattern.
The resolving power W of an exposure apparatus, which depends on the wavelength λ of exposure light and the numerical aperture NA of the projection optical system, is represented by the following expression (a):W=K·λ/NA (K: constant)  (a)
Therefore, for improving the resolving power of the exposure apparatus, it is necessary to shorten the wavelength λ of exposure light or increase the numerical aperture NA of the projection optical system. Since it is difficult for the numerical aperture NA of the projection optical system to be a predetermined value or greater from the viewpoint of optical designs in general, it will be necessary for the exposure light to have a shorter wavelength from now on. For example, resolving powers of 0.25 μm and 0.18 μm are obtained when a KrF excimer laser having a wavelength of 248 nm and an ArF excimer laser having a wavelength of 193 nm are used as the exposure light, respectively. The resolving power can further be improved when X-rays having a wavelength shorter than these laser beams are used as the exposure light. For example, a resolving power of 0.1 μm or finer is obtained when an X-ray having a wavelength of 13 nm is used as the exposure light.
Meanwhile, since there are no usable transmissive optical materials and refractive optical materials when X-rays are used as the exposure light, a reflective projection optical system is used together with a reflective mask. Projection optical systems employable in exposure apparatus using X-rays as the exposure light have conventionally been disclosed in U.S. Pat. No. 5,815,310, its corresponding Japanese Patent Application Laid-Open No. H 9-211322, and U.S. Pat. No. 6,183,095B1, for example. Reflective optical systems each using eight reflectors have been disclosed in various modes in U.S. Pat. No. 5,686,728, its corresponding Japanese Patent Application Laid-Open No. H 10-90602, Japanese Patent Application Laid-Open No.2002-139672, and the like.
The foregoing will be summarized as:
Patent Document 1: U.S. Pat. No. 5,815,310
Patent Document 2: Japanese Patent Application Laid-Open No. H9-211322                Patent Document 3: U.S. Pat. No. 6,183,095B 1        Patent Document 4: U.S. Pat. No. 5,686,728        Patent Document 5: Japanese Patent Application Laid-Open No. H10-90602        Patent Document 6: Japanese Patent Application Laid-Open No. 2002-139672        
The conventional reflective optical system disclosed in Japanese Patent Application Laid-Open No. H9-211322 is a relatively bright optical system comprising six reflectors and having a numerical aperture of 0.25, but fails to achieve a brighter NA.
The reflective optical system in accordance with the first embodiment described in U.S. Pat. No. 6,183,095B1 is a relatively bright optical system comprising six reflectors and having a numerical aperture NA of 0.25, but fails to achieve a brighter NA, either. A reflective optical system constructed by eight reflectors has been known from U.S. Pat. No. 5,686,728. Though this reflective optical system is made compact while securing a large numerical aperture (NA) of 0.3 or greater, the beam incidence angle at each surface of the eight reflectors is 40° to 50°, which is too large. This makes it difficult to design a reflective multilayer film, and is disadvantageous in terms of securing accuracy at the time of manufacture and stability when performing the projection.
In the reflective optical systems in accordance with the embodiments described in Japanese Patent Application Laid-Open No. 2002-139672, the curvatures of the reflectors are too large, though the angles of incidence of beams at the reflectors are not so large. Therefore, there are no effective means for inspecting errors in forms of aspherical reflectors, whereby the inspection is difficult.
The currently most effective inspecting means is a method known as PDI. The PDI is a means for inspecting a reflector by making light passing through a pinhole reflect with the reflector. In the case where an inspection is performed by the PDI, a higher inspection accuracy and a better effect are obtained when the absolute value of radius of curvature of each reflector is relatively small.